The present invention pertains to the vacuum tube arts, and in particular to an x-ray tube cathode cup structure for deflecting a focal spot of a beam of electrons. It finds particular application in conjunction with rotating anode x-ray tubes for CT scanners and will be described with particular reference thereto. However, it is to be appreciated that the present invention will also find application in the generation of radiation and in vacuum tubes for other applications.
Conventional x-ray tubes include a vacuum enclosure and a source of a beam of electrons in the form of a cathode. The cathode includes a heated filament which emits electrons. The impact of the electron beam on the anode causes a beam of x-radiation to be emitted from the x-ray tube, typically through a beryllium window. A trend toward shorter x-ray exposure times in radiography has placed an emphasis on having a greater intensity of radiation and hence higher electron currents. Increasing the intensity can cause overheating of the x-ray tube anode. An electrical bias voltage is applied to the beam of electrons in order to control, to some extent, the size of the focal spot.
One way to control the size of the focal spot of the electrons on the anode more closely is to mount the cathode filament within a cathode focusing or support cup member. Such a system is shown in U.S. Pat. No. 4,689,809. A cathode cup is split into two portions, surrounding the filament, The portions are biased equal to or negative with respect to the filament. The biased cup reduces unwanted xe2x80x9cwings,xe2x80x9d or diffused areas, appearing as part of the x-ray focal spot.
Other cathode cup and filament arrangements for controlling the size and shape of the electron focal spot on the tube anode are discussed in U.S. Pat. Nos. 4,685,118, 5,224,143, and 5,065,420.
To minimize the power requirements of the focussing system and to maintain accurate positioning of the filament relative to the deflectors, it is desirable to mount both the deflectors and the filament to the same support. Cathode cups thus typically include a base or arm portion which supports the filament and a pair of deflectors. The deflectors are mechanically mounted to the base, but are electrically insulated from it. This is achieved through the use of ceramic insulators which are brazed to both the base and the deflectors in the form of a sandwich. The ceramic insulators include central bores through which a bolt is received for maintaining alignment of the components during brazing. To avoid shorting, the bolt is electrically isolated from the base. Such a cathode cup design is difficult to assemble, difficult to align, and is susceptible to shorting. This can occur if the material used to braze the ceramic insulator to the base or the deflector flows into the insulator bore that receives the bolt. Shorting can also occur due to natural plating of the ceramic insulator with metal vapor from the filament.
The present invention provides a new and improved x-ray tube and method which overcomes the above-referenced problems and others.
In accordance with one aspect of the present invention, a cathode assembly is provided. The assembly includes a base. A filament is mounted to the base for delivering a stream of electrons. A deflector is carried by the base for deflecting the electrons and/or focusing the electrons into a beam. An insulator electrically insulates the deflector from the base. The insulator defines a bore. A rod is connected with the deflector adjacent a first end of the rod. The rod is received within the insulator bore.
In accordance with another aspect of the present invention, an x ray tube is provided. The x-ray tube includes an envelope which encloses an evacuated chamber. A cathode assembly is disposed within the chamber for providing a source of electrons. The cathode assembly includes a base supported in the envelope. A filament is mounted to the base for providing the electrons. A deflector is carried by the base for deflecting the electrons and/or focusing the electrons into a beam. An insulator electrically insulates the deflector from the base. The insulator defines a bore. A rod is connected with the deflector adjacent a first end of the rod, the rod being received within the insulator bore. An anode is disposed within the chamber and positioned to be struck by the electrons and generate x-rays.
In accordance with another aspect of the present invention, a method of assembling a cathode assembly is provided. The method includes attaching at least one rod to at least one deflector and attaching a metal tube in an insulator to define a bore for receiving the rod. The insulator is attached to a base. A filament assembly is attached to the base. The method further includes sliding the rod into the tube to mount the deflector to the base and attaching the rod to the tube.
One advantage of at least one embodiment of the present invention is that a cathode cup is electrically isolated from a filament.
Another advantage of at least one embodiment of the present invention is that deflectors of a cathode cup are readily aligned with a filament.
Another advantage of at least one embodiment of the present invention is that components of a cathode cup are accurately aligned.
Another advantage of at least one embodiment of the present invention is that deposition of vaporized filament material on to insulators which space the deflectors from a base assembly is minimized by reducing the line of sight between the filament and the insulators.
Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.